The scope of the present invention is not limited to Blu-Ray Discs (BD) but the following description assumes use of the BD and uses the same terminology as in the BD field.
Most optical disk devices including BD use a high-frequency modulation method to limit noise generated by the laser diode that is utilized as the light source. This technology is disclosed in “Kogaku” Vol. 14, No. 5, pp. 377-383. Since this technology is well known in this field, only essential matters are described below and other matters are omitted.
The oscillation of the laser diode becomes unstable when laser beam reflected from the disk enters the laser diode during oscillation and consequently generates significant laser noise. The high-frequency modulation method is utilized to avoid this laser noise. This technique is called the high-frequency modulation method because a high-frequency signal is superimposed onto the laser diode drive signal to make the laser emit pulsed laser beam. The light in this light waveform is repeatedly turned on and off as shown in FIG. 2. The ratio (duty) of the laser pulse interval (modulation cycle) and light emission period for that laser pulse period here is a parameter for adjusting laser noise to a minimum. In other words, the frequency and the duty are selected so that the laser pulse reflected from the disk does not enter the laser diode while the laser is oscillating.
The laser beam waveform appears as shown in FIG. 2, so the read signal waveform will appear as shown in FIG. 3, assuming that there are no bandwidth limitations from the photodiode and current-to-voltage converter amplifier used for reading. Such pulsed signal made of read pulse train is hereafter called the pulsed read signal. The broken line in FIG. 3 is the read signal waveform obtained assuming consecutive oscillation at the same output power that of the laser pulse peak power when the high-frequency carrier is superimposed. In other words, the contour of the upper envelope of the pulsed read signal is same as a read signal waveform obtained by consecutive light. The desired read waveform can therefore be obtained by envelope detection, namely by passing the pulsed read signal through a low-pass filter with a cut-off frequency that is sufficiently lower than the frequency of the superimposed high-frequency current. In modern optical disk devices, these functions are implemented by bandwidth limitation by circuits made up of photo-detectors and current-to-voltage converter amplifiers, and analog equalizers.
FIG. 6 shows an example (thick curve) of the pulsed read signal spectrum. The frequency of the superimposed high-frequency signal is 400 MHz and the pulse duty is 0.2. The component near the direct current region is the consecutive read signal. Generation of a pulsed read signal is a sort of amplitude modulation and therefore line-like spectrum of the superimposed high-frequency signal and, the modulated read signal component near that line-like spectrum can be observed. The superimposed high-frequency signal is therefore simply called as carrier hereafter.
The most common carrier frequency may for example be 400 MHz in the case of BD. There should be small carrier frequency difference among such devices because it is determined by the optical path length in the read-optical system.
The read speed of optical disk is limited by the rotation speed of the disk if the linear recording density is a fixed value. The maximum disk rotation speed attainable is limited by the strength of the disk and in the case of polycarbonate disks with a 12 centimeter diameter is approximately 10,000 rpm (revolutions per minute). As is common knowledge to those skilled in the art, there is a high probability that disks rotating at a higher speed will break. The maximum speed attainable by BD is therefore 12× speed. The maximum read speed of practical consumer optical disk drives as of 2007 is 6× BD drive.